Method of equalizing the voltage across drive batteries connected in series during recharging, for electric vehicles, and a device for implementing the method

Abstract: In order to equalize the voltages of batteries ( 1 to n) connected in series during recharging, the voltage (V₁ to V n ) and the temperature (T₁ to T n ) of each battery are measured and only the battery with the lowest voltage is charged with higher-intensity current, a current bypass being applied to the other batteries; the steps are repeated until all the batteries (1 to n) have reched the same voltage in relation to the temperature measured.

Description: The present invention relates to a method of equalizing the voltages across drive batteries connected in series during recharging, for electric vehicles, and to a device for implementing the method. In order to operate, electric vehicles require a large number of drive batteries which are connected in series and, at the moment, are usually lead-acid or lead-gel (recombination) batteries. Whatever type of battery is used, the autonomy of the vehicle is fairly limited and it is therefore necessary to provide for very frequent recharging of the batteries by means of a battery charger which may be in the vehicle or on the ground and which supplies electrical energy to the batteries, taking it from the mains. Because they are arranged in series in the vehicle, and because of the difficulty of charging them in parallel (in fact, it would be necessary to have access to each individual battery), recharging is effected in series, and the charging current is therefore the same for all the batteries. It is also known that there are small voltage differences between one battery and another; these differences may be intrinsic in the construction of the batteries themselves or may be due to different storage periods before fitting in the vehicle, or simply to different temperatures from one battery to another. Clearly, if charging is carried out in series, even small voltage differences between the various batteries are not compensated, on the contrary they will be increased with each recharging cycle. It is also known that a battery which is overcharged or undercharged in comparison with the others will quickly be damaged, completely jeopardizing the use of the vehicle in which it is fitted. Methods of equalizing the voltage across drive batteries connected in series are known from WO-A-93 08 629 and US-A-4 238 721. The object of the present invention is to provide a method of equalizing the voltages across drive batteries connected in series during recharging, for electric vehicles, which enables the voltage in all the batteries making up the series to be equalized. The present invention achieves the aforesaid object by virtue of a method of equalizing the voltage across drive batteries connected in series during recharging, for electric vehicles according to claim 1. The invention also comprises a device for implementing the method given above, having the characteristics specifically referred to in the following claims 2-4. The invention will now be described by way of non-limiting example, with reference to the appended drawings, in which: Figure 1 is a flow-chart of the method according to the invention, Figure 2 is a block diagram of the device which implements the method of the invention, and Figure 3 shows the structural details of a portion of Figure 2. With reference to Figure 1, in step a), the recharging of the batteries is started. In step b), the voltage and the temperature values of each individual battery are measured and stored and a standardized voltage obtained by applying, to the corresponding voltage measured, a correction factor depending upon the respective temperature measured is derived and stored for each battery. In step c), the various standardized voltages are compared; if all the batteries have reached the same standardized voltage, the recharging of the batteries is continued according to normal techniques, except for continuous voltage checks and resumption of the equalization procedure, if necessary; if this situation is not confirmed (step d)), it is determined to which battery/ies the lowest standardized voltage corresponds and a current bypass circuit is applied to all the batteries which have standardized voltages higher than the lowest standardized voltage; after a predetermined period of time, all the bypasses are disconnected and all the steps b) to d) are repeated until all the batteries have reached the same standardized voltage. With reference to Figure 2, n batteries, indicated 1 to n, are connected in series for driving an electric vehicle, not shown. The positive pole of the battery 1 and the negative pole of the battery n are connected to the electric drive motor (not shown). A battery charger 8 has its positive pole connected to the positive pole of the battery 1 and its negative pole connected to the negative pole of the battery n. The positive poles of the batteries 1 to n are connected, respectively, to the first inputs 10 1 ... 10 n of cells 12 1 ... 12 n ; the first inputs 10 1 ... 10 n are connected, respectively, to second inputs 14 1 ... 14 n of the cells 12 1 ... 12 n by means of respective switches 16 1 ... 16 n . The cells 12 1 to 12 n-1 have respective first outputs 18 1 ... 18 n-1 each connected to the respective first input 10 2 ... 10 n of the immediately following cell, the first output 18 n of the cell 12 n being connected to the negative pole of the battery n. The first inputs of the cells 12 1 ... 12 n are connected to respective switches 20 1 ... 20 n while the first output 18 n of the cell 12 n is connected to a switch 20 n+1 . Each cell 12 1 to 12 n has a respective second output 31 1 to 31 n and a respective third output 32 1 to 32 n , which are connected, respectively, to the positive and negative inputs of the current-stabilizer 8. A microprocessor 50 controls the switches 20 1 to 20 n+1 by means of control cables 52 and receives the signals coming from the first inputs 10 1 to 10 n of the cells 12 1 to 12 n when the switches 20 1 to 20 n+1 are closed. Third inputs 54 1 to 54 n and fourth inputs 56 1 to 56 n of each individual cell 12 1 to 12 n are connected to the outputs 58 and 60 respectively of the microprocessor 50. The switches 16 1 to 16 n are connected, respectively, to outputs 60 1 to 60 n of the microprocessor 50 to the inputs 62 1 to 62 n of which temperature sensors 64 1 to 64 n on the batteries 1 to n are connected, an output 66 of the microprocessor 50 being connected to the input 68 of the battery charger 8. One of the cells 12 1 to 12 n , and more precisely the cell 12 1 , will now be described with reference to Figure 3, the description applying to the other cells in an identical manner. In Figure 3, an opto-isolator, indicated 105, receives the chopper signal which comes from the outputs 58, 60 of the microprocessor (not shown in this drawing ) which controls a driver 107 for driving a MOSFET circuit 109. The opto-isolator 105, the driver 107 and the MOSFET 109 are connected to the negative terminal of the associated battery 1 and the opto-isolator 105 and the driver 107 are also connected to a first contact of the associated switch 16 1 for the on-off control of the cell 12 1 . The MOSFET 109 is connected to the primary winding 110 of a transformer 111, the primary winding 110 being connected to the negative pole of the battery 1 by means of a capacitor 112 and to the positive pole of the battery 1 by means of a rectifier 113; the anode of the rectifier 113, which is connected to the positive pole of the battery 1, being connected to the associated switch 20 1 in order to measure the voltage of the battery 1. The secondary winding 120 of the transformer 111 is connected to the two poles of the battery charger (not shown in this drawing); the connection to the positive pole is made with the interposition of a rectifier 122; there is a capacitor 124 between the cathode of the rectifier 122 and the connection to the negative pole of the battery charger. In use, assuming that the batteries 1 to n are at the recharging stage, the charging of the batteries is started by means of the battery charger 8; all the switches 16 1 to 16 n and 20 1 to 20 n are open. The voltage across each battery is then measured and stored in the microprocessor 50, for which purpose the microprocessor 50 causes the pair of switches 20 1 and 20 2 to close, thus measuring the voltage V 1 across the battery 1, this voltage value being stored in the microprocessor 50; the pair of switches 20 2 and 20 3 is then closed, thus measuring the voltage V 2 across the battery 2, and so on up to the pair 20 n , 20 n+1 , measuring and storing the voltage V n . Upon completion of the measurement of the voltages V 1 to V n , all the switches 20 1 to 20 n+1 are open. Simultaneously with the measurement and storage of the voltages V 1 to V n , the instantaneous temperatures T 1 to T n of the batteries 1 to n are measured and stored in the microprocessor 50 by means of the sensors 64 1 to 64 n . The microprocessor 50 derives and stores, for each battery 1 to n, a standardized voltage obtained by applying to the respective voltage V 1 to V n measured, a correction factor depending upon the respective temperature T 1 to T n measured; the microprocessor 50 then checks whether all the batteries 1 to n have the same standardized voltage and, if so, the equalizing procedure ends, as will be described below; if not, the microprocessor 50 compares the standardized voltages and determines which is the lowest; it then limits the charging current to all the batteries with standardized voltages higher than the minimum standardized voltage. In order to do this, the microprocessor 50 closes the switches 16 1 to 16 n corresponding to the batteries to which the charging current is to be limited, forming a current bypass for these batteries, leaving open only the switch/es corresponding to the battery/ies with the lowest voltage. In this situation, the cells 12 1 to 12 n of which the corresponding switches 16 1 to 16 n are closed absorb current, in practice acting as shunts for the associated batteries. The cells 12 1 to 12 n may be of the purely resistive type and, in this case, simply absorb current, transforming it into heat by the Joule effect, or, preferably are of the active type as will be described below, in which some of the current absorbed is restored to the battery charger 8 by means of the outputs 31 1 to 31 n and 32 1 to 32 n . The microprocessor 50 pilots the battery charger 8 by means of the connection formed therewith between the terminals 66 and 68, so that it draws from the mains only the portion of current necessary to achieve the nominal charging current, taking account of the current contribution supplied to the battery charger 8 by the cells 12 1 ... 12 n to which the bypass has been applied. The situation just described lasts for a predetermined period of time and when it has elapsed, the microprocessor 50 eliminates all the bypasses, opening all the switches 16 1 to 16 n . The microprocessor 50 then repeats all the steps just described, in order, until all the batteries have reached the same standardized voltage, after which the voltage-equalization procedure is completed and the recharging can take place according to normal techniques, except for continuous voltage checks and resumption of the equalization procedure, if necessary. The operation of the cell 12 1 will be described with reference to Figure 3, the description applying to all the other cells in an identical manner. When the switch 16 1 is open, no current circulates in the cell 12 1 . If this switch is closed, the MOSFET 109 changes cyclically from the cut-off to the saturation condition, driven by the chopper signal coming from the microprocessor 50 by means of the connections 58 and 60, and an alternating current is produced in the secondary winding 120 of the transformer 111, which current is rectified by the rectifier 122, and is smoothed by the capacitor 124. This current is sent to the battery charger, achieving an energy saving since the battery charger 8, which is piloted by the microprocessor 50, will require from the mains only the difference between amount which is nominally necessary and the amount restored from the cell 12 1 . Naturally, the principle of the invention remaining the same, the details of construction and forms of embodiment may be varied widely with respect to those described and illustrated, without thereby departing from the scope of the present invention as defined by the appended claims. In particular, the circuit components of the cells 12 1 to 12 n could be varied widely, the switches 16 1 to 16 n and the switches 20 1 to 20 n+1 could be incorporated in the cells 12 1 to 12 n , and the microprocessor 50 could be incorporated in the battery charger 8, or could be the microprocessor already present in the vehicle.

Claims: A method of equalizing the voltages across drive batteries connected in series during recharging, for electric vehicles, by means of a battery charger connected to the electricity-supply mains outside the vehicle, and having its positive output connected to the positive terminal of the first battery of the series and its negative output connected to the negative terminal of the last battery of the series, comprising the following steps, in succession and in order: a) beginning the recharging of the batteries; b) measuring and storing the voltage (V 1 to V n ) across each battery (1 to n) and its temperature (T 1 to T n ); c) deriving and storing, for each battery (1 to n), a standardized voltage obtained by applying, to the respective voltage (V 1 to V n ) measured, a correction factor depending upon the respective temperature (T 1 to T n ) measured characterized in that it further comprises the following steps in succession and in order: d) determining to which battery/ies the lowest standardized voltage corresponds, e) applying a current bypass to all the batteries (1 to n) with standardized voltages higher than the lowest standardized voltage; f) continuing to recharge the batteries (1 to n) for a predetermined period of time; g) eliminating all the bypasses applied; h) repeating steps b) to g) until all the batteries (1 to n) have equal standardized voltage values and then terminating the equalization procedure. A device for implementing the method according to Claim 1, in which a battery charger (8) recharges a plurality of batteries (1 to n) connected in series, the battery charger (8) having its positive output connected to the positive terminal of the first battery of the series and its negative output connected to the negative terminal of the last battery of the series, comprising a microprocessor (50) for piloting the battery charger; means (20 1 to 20 n+1 ) and (64 1 to 64 n ) piloted by the microprocessor (50) for enabling the microprocessor (50) to measure the voltage (V 1 to V n ) across each battery (1 to n) and its temperature (T 1 to T n ), respectively, in order to store the voltage and the temperature in the microprocessor (50), the microprocessor (50) being arranged to derive and store, for each battery (1 to n), a standardized voltage obtained by applying to the respective voltage (V 1 to V n ) measured a correction factor depending upon the respective temperature (T 1 to T n ) measured characterized in that the microprocessor (50) is arranged to determine to which battery/ies the lowest standardized voltage corresponds, and in that it further comprises: means (16 1 to 16 n ; 12 1 to 12 n ) piloted by the microprocessor (50) for applying a current bypass to the batteries with standardized voltage values higher than the lowest standardized voltage value, enabling only the battery/ies with the lowest standardized voltage to be recharged with a higher current. A device according to Claim 2, characterized in that the current bypass is achieved by diverting the charging current into active cells (12 1 to 12 n ) which can recover some of the current supplied by the battery charger (8). A device according to Claim 3, characterized in that each active cell (12 1 to 12 n ) comprises an opto-isolator (105) connected to the outputs (58, 60) of the microprocessor (50), in order to receive therefrom a chopper signal, the opto-isolator (105) being connected to a driver (107) connected to a MOSFET circuit (109); the opto-isolator (105), the driver (107), and the MOSFET (109) being connected to the negative terminal of the associated battery (1); the opto-isolator (105) and the driver (107) being connected to the first contact of the associated switch (16 1 ) for the on-off control of the cell (12 1 ), the MOSFET (109) being connected to the primary winding (110) of a transformer (111), the primary winding (110) being connected to the negative pole of the battery (1) by means of a capacitor (112) and to the positive pole of the battery (1) by means of a rectifier (113); the anode of the rectifier (113) being connected to the positive pole of the battery (1) and to the associated switch (20 1 ) in order to measure the voltage of the battery (1); the secondary winding (120) of the transformer (111) being connected to the positive pole of the battery charger (8) with the interposition of a rectifier (122); and a capacitor (124) being connected between the cathode of the rectifier (122) and the negative pole of the battery charger (8).

Inventors: GUERZONI ROBERTO, VARRONE PIERGIORGIO, GUERZONI, ROBERTO, VARRONE, PIERGIORGIO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1994
Main classification: H02J7
IPC codes: H02J7